The present invention relates to the field of spectroscopic detectors. Specifically, the present invention relates to a hemispherical detector for use with a transmittance or reflectance spectrometer which comprises a plurality of photodetectors.
Infrared spectroscopy is a technique which is based upon the vibrations of the atoms of a molecule. In accordance with infrared spectroscopy, an infrared spectrum is generated by transmitting radiation through a sample and determining what portion of the incident radiation is absorbed by the sample at a particular energy. Near infrared radiation is radiation having a wavelength between about 700 nm and about 2500 nm.
In general spectrometers (e.g., a spectrophotometer) can be divided into two classes: transmittance spectrometers and reflectance spectrometers. In a transmittance spectrometer, light having a desired narrow band of wavelengths is directed onto a sample, and a detector detects the light which was transmitted through the sample. In contrast, in a reflectance spectrometer, light having a narrow band of wavelengths is directed onto a sample and one or more detectors detect the light which was reflected off of the sample. Depending upon its design, a spectrometer may, or may not, be used as both a transmittance and a reflectance spectrometer.
A variety of different types of spectrometers are known in the art such as grating spectrometers, FT (Fourier transformation) spectrometers, Hadamard transformation spectrometers, AOTF (Acousto Optic Tunable Filter) spectrometers, multiple discrete wavelength source spectrometers, filter-type spectrometers, scanning dispersive spectrometers, and double-beam spectrometers.
Filter-type spectrometers, for example, utilize a light source (such as a conventional light bulb) to illuminate a rotating opaque disk, wherein the disk includes a number of narrow bandpass optical filters. The disk is then rotated so that each of the narrow bandpass filters passes between the light source and the sample. An encoder indicates which optical filter is presently under the light source. The filters filter the light from the light source so that only a narrow selected wavelength range passes through the filter to the sample. Optical detectors are positioned to detect light which either is reflected by the sample (to obtain a reflectance spectra) or is transmitted through the sample (to generate a transmittance spectra). The amount of detected light is then measured, which provides an indication of the amount of absorbance of the light by the substance under analysis.
Multiple discrete wavelength source spectrometers use infrared emitting diodes (IREDs) as sources of near-infrared radiation. A plurality (for example, eight) of IREDs are arranged over a sample work surface to be illuminated for quantitative analysis. Near-infrared radiation emitted from each IRED impinges upon an accompanying optical filter. Each optical filter is a narrow bandpass filter which passes NIR radiation at a different wavelength. NIR radiation passing through the sample is detected by a detector (such as a silicon photodetector). The amount of detected light is then measured, which provides an indication of the amount of absorbance of the light by the substance under analysis. IRED reflectance spectroscopy is also possible.
Acousto Optic Tunable Filter spectrometers utilize an RF signal to generate acoustic waves in a TeO2 crystal. A light source transmits a beam of light through the crystal, and the interaction between the crystal and the RF signal splits the beam of light into three beams: a center beam of unaltered white light and two beams of monochromatic and orthogonally polarized light. A sample is placed in the path of one of the monochromatic beams and detectors are positioned to detect light which either is reflected by the sample (to obtain a reflectance spectra) or is transmitted through the sample (to generate a transmittance spectra). The wavelength of the light source is incremented across a wavelength band of interest by varying the RF frequency. The amount of detected light is then measured, which provides an indication of the amount of absorbance of the light by the substance under analysis.
In grating monochrometer spectrometers, a light source transmits a beam of light through an entrance slit and onto a grating element (the dispersive element) to disperse the light beam into a plurality of beams of different wavelengths (i.e., a dispersed spectrum). The dispersed light is then reflected back through an exit slit on to a detector. By selectively altering the path of the dispersed spectrum relative to the exit slit, the wavelength of the light directed to the detector can be varied. The amount of detected light is then measured, which provides an indication of the amount of absorbance of the light by the substance under analysis. The width of the entrance and exit slits can be varied to compensate for any variation of the source energy with wavenumber. This approach lends itself to reflectance spectrometry.
Dual-beam spectrometers split radiation from a source into two beams, half passing into a sample-cell compartment and the other half into a reference area. The reference beam then passes through an attenuator and on to a chopper, which is a motor-driven disk that alternatively reflects the reference or transmits the beam to a detector. After dispersion by a prism or grating, the sample-cell beam passes to the sample and a detector detects the transmittance that passes through the sample or reflectance that reflects from the sample. If the two beams are identical in power, the detectors transmit similar electrical signals to a null detector. The null detector in turn produces an unfluctuating direct current. However, if the two beams differ in power, the detectors transmit differing electrical signals to the null detector. In this case, the null detector produces a fluctuating electrical current, which is used to generate the spectral data. For example, the fluctuating current can be used to drive a synchronous motor in one direction or the other depending upon the phase of the current; with the synchronous motor mechanically linked to a pen drive of a recorder, which the synchronous motor causes to move to generate the spectral data. This approach lends itself to both transmittance and reflectance spectrometry.
Detectors used in spectroscopy generally fall into two classes, photographic detectors, in which radiation impinges upon an unexposed photographic film, and electronic detectors, in which the radiation impinges upon a detector and is converted into an electrical signal. Electronic detectors provide the advantage of increased speed and accuracy, as well as the ability to convert the spectral data into an electronic format, which can be displayed, processed, and/or stored. Examples of electronic detectors include photomultiplier tubes and photodetectors. Photomultiplier tubes are quite sensitive, but are relatively large and expensive. Photodetectors provide the advantage of reduced size and cost. These detectors include IR detectors, pin diode detectors, charge coupled device detectors, and charge injection device detectors.
Conventionally, spectroscopic detectors are configured either as a single detector, flat detector, or a plurality of discrete detectors arranged in common plane (e.g. a flat array). In either case, these xe2x80x9cflatxe2x80x9d detector arrangements inherently detect only a 3% portion of the transmitted or reflected spectral data for 1 cm2 detectors at a 2 cm distance from the source detector.
As described in Bums and Ciurczak, HANDBOOK OF NEAR-INFRARED ANALYSIS, pp 42-43 (1992), detectors for measuring diffuse reflectance are known which include either two or four opposing detectors arranged at a 45 degree angle from the sample. In general, PbS detectors are used for measurements in the 1100-2500-nm region, whereas PbS xe2x80x9csandwichedxe2x80x9d with silicon photodiodes are most often used for visible-near-infrared applications (typically 400-2600 nm).
The signal from the detectors is added to a low-noise, high-gain amplifier and then converted from analog to digital. The digital signal is exported from the instrument to an on-board or external microcomputer for data processing, calibration, and storage. The computer records a signal representing the actual wave-length used for measurement with the raw reflectance or transmittance digital data. This function is repeated for both the sample and the reference. The spectrum, then, is the difference between the raw reflectance measurement of the sample and the raw reflectance measurement of the reference material. Raw reflectance is converted to absorbance using the function Absorbance=xe2x88x92log (10)* Reflectance, commonly referred to as log 1/R. Raw transmittance is converted to absorbance using the expression log 1/T.
When configured with four opposing 1 cm2 detectors at 45 degree angles and 2 cm from the sample, the diffuse reflectance detector described above provides the advantage of collecting spectral data from four different vantage points, as compared to more conventional xe2x80x9cflatxe2x80x9d detector arrangements. However, even with this configuration, only about 12% of the reflected spectral data is detected. Moreover, this configuration is not suitable for use with a transmittance spectrometer.
In accordance with a first embodiment of the present invention, a hemispherical detector for use with a transmittance spectrometer is provided which comprises a plurality of photodetectors arranged in a substantially contiguous array, the array being substantially in the shape of a half-sphere, the half-sphere defining a closed end and an open end, the open end defining a substantially circular face. In use, a sample to be analyzed preferably intersects a plane passing through the substantially circular face, and a transmitted beam of light from the transmittance spectrometer intersects the plane at a 90 degree angle to, and at a center-point of, said substantially circular face. In this manner, substantially all of the light which passes through the sample is detected by the detector array. Currently, most photodetectors have a flat surface. Therefore, the individual photodetectors which comprise the array of photodetectors are preferably about 0.5-3 mm2 in order to provide a substantially spherical shape. If available, smaller photodetectors can also be used. In this manner, except for beams of light which strike between photodetectors, all of the light which passes through the sample is detected by the photodetector array. In this regard, it is believed that this configuration can detect about 80% of the spectral data.
In accordance with a second embodiment of the present invention, a hemispherical detector for use with a reflectance spectrometer is provided which comprises a plurality of photodetectors arranged in a substantially contiguous array, the array being substantially in the shape of a truncated half-sphere, the truncated half-sphere defining a first open end and a second open end, the second open end defining a substantially circular face having a diameter (xe2x80x9cdxe2x80x9d), the first open end having a cutout formed therein, wherein the cutout defines an area which is less than Π (d/2)2. In use, a sample to be analyzed preferably intersects a plane passing through the substantially circular face, and a transmitted beam of light from the reflectance spectrometer passes through the second open end in a direction perpendicular to the plane and co-axial with a center-point of said substantially circular face. In this manner, substantially all of the light which reflects off of the sample is detected by the detector array. As with the first embodiment described above, the individual photodetectors which comprise the array of photodetectors are preferably about 0.5-3 mm2 in order to provide a substantially spherical shape. In this manner, except for beams of light which strike between photodetectors, or are reflected back through the first open end, all of the light which is reflected off of the sample is detected by the photodetector array. Preferably, the area of the opening defined by the first open end is minimized in order to maximize the percentage of the reflected light which is received by the detector arrays. However, the opening must remain sufficiently large to allow the transmitted beam of light to impinge upon the sample. Also, motion due to the operation of the spectrometer may cause the shell of the detector to infringe the path of the beam of light. Most preferably, the first open end is a circular cut-out having a diameter of approximately 5 mm.
In accordance with a third embodiment of the present invention, a hemispherical detector for use with a reflectance or transmittance spectrometer is provided which comprises a plurality of photodetectors arranged in a substantially contiguous array, the array being substantially in the shape of a half-sphere. The half-sphere includes a first portion and a second portion. The first portion is in the shape of a truncated half sphere, the truncated half sphere defining a first open end and a second open end, the second open end defining a substantially circular face having a diameter (xe2x80x9cdxe2x80x9d), the first open end having a cutout formed therein, wherein the cutout defines an area which is less than Π (d/2)2. The second portion is removably secured to the first open end. When performing a transmittance measurement, the second portion is secured to the first portion, thereby forming photodetector array which is substantially in the shape of a half-sphere. The hemispherical detector can then be used in the manner described above with reference to the first embodiment. In order to perform a reflectance measurement, the second portion is removed from the first portion, thereby forming photodetector array which is substantially in the shape of a truncated half-sphere. The hemispherical detector can then be used in the manner described above with reference to the second embodiment.
It is believed that the detectors of the second and third embodiment, like the first embodiment, can detect approximately 80% of the spectral data.
In each of the embodiments described above, openings are preferably provided in the shell to allow wires to contact the photodetectors. This prevents the wires from interfering with data acquisition.
The detectors in accordance with the present invention may be used in a variety of spectrometers including, for example, filter-type spectrometers, multiple discrete wavelength source spectrometers, AOTF (Acousto Optic Tunable Filter) spectrometers, grating spectrometers, FT (Fourier transformation) spectrometers, Hadamard transformation spectrometers, post-dispersive monochrometer spectrometers, double beam spectrometers, and scanning dispersive spectrometers. In these embodiments the detectors provide the advantage of a more accurate measurement by increasing the percentage of spectral data which is detected.
In the embodiments described above, the substantially circular face preferably has a diameter of from about 1.5 mm to about 1 m.
The hemispherical detectors in accordance with the present invention may be constructed in a number of ways.
For example, the hemispherical detector may be constructed by a mold method. In accordance with this method, a press mold is created, a material is poured into the mold to create a cast (which forms the shell of the detector) and a plurality of photodetectors are attached to the cast. This has the advantage of quick and efficient construction. Moreover, a plurality of uniform hemispherical detectors may be made. The cast preferably has a diameter of from about 1.5 mm to about 1 m.
The hemispherical detector may also be constructed by an airform method. In accordance with this method, a malleable airform, e.g., plastic, may be fabricated to the proper shape and size, inflated, and then coated with a hardening material to create the shell of the detector. This has the advantage of a strong and stable hemispherical detector at a marginal cost. Also, this method provides the advantage that detectors of differing sizes can easily be constructed by modifying the amount of material in the airform. The malleable airform preferably has a diameter of from about 1.5 mm to about 1 m.
The hemispherical detector may also be constructed by geodesic dome method. In accordance with this method, a plurality of pentagons, hexagons, and half hexagons are joined together in a geodesic dome shape, e.g., such that every pentagon is surrounded by 5 hexagons, half-hexagons, or combination thereof. Photodetectors or fillings with photodetectors attached may be secured in the areas between the struts. This has the advantage of a versatile and sturdy construction. The geodesic dome shape preferably has a diameter of from about 1.5 mm to 1 m. Moreover, as six struts could form the entire circumferential length of the dome, each strut preferably has a length of from about 0.39 mm to 0.26 m.
Preferably, the ceramic mold, airform, or geodesic dome hemispherical detector construction method may be further modified to allow for additional wiring. In this regard, apertures may be drilled in the hemispherical detector to allow wiring to contact the photodetector.
Although the above-referenced methods of construction are preferred, other methods known in the art may alternatively be used.